"Tourism is not regulated well enough and the massive influx is putting a strain on sites all over the world," says Stefaan Poortman, executive director of the Global Heritage Fund. "The most well-known sites like Angkor or Machu Picchu are really swamped with tourism."
Great Wall of China —
It was originally built to defend an empire, but now parts of the Great Wall of China are crumbling so badly they need someone to leap to their defense. About 2,000 kilometers, or 30%, of the ancient fortification built in the Ming Dynasty era has disappeared due to natural erosion and human damage, according to the Beijing Times.
Great Barrier Reef —
The world's largest coral reef ecosystem hosts scores of marine species, but scientists say that it could become extinct as soon as 2050 due to climate change.
Tanya Puntti / Shutterstock
Angkor Wat —
Cambodia's Angkor Wat was visited by more than two million people in 2013. One of the biggest issues facing the Angkor Archaeological Park is that a few major sites -- namely Angkor Wat, Angkor Thom and the Bayon -- endure the bulk of tourist traffic.
TANG CHHIN SOTHY/AFP/Getty Images
Sistine Chapel —
The ceiling of the Vatican's Sistine Chapel, painted by Michelangelo, attracts millions of tourists each year. Last year a new heating and cooling system was introduced in order to try to combat the harmful side effects of tourism.